The invention relates to a method, a control device and aids for the achievement of uniform printing results on an autotypically working color offset printing machine. In such machine the feed of printing inks to adjacent color zones of a printing substrate is adjustable by means of regulators and for the regulation of the printing process, solid densities and/or screen dot sizes are determined repeatedly on measuring patches simultaneously printed within the color zones, and, when they fall outside of tolerances associated with them, intervention is made correctively in the printing process.
Polychrome originals are today reproduced mostly by a four-color printing process wherein four primary colors, usually cyan, magenta, yellow and black are used. The originals are first broken down into so-called color separations, which are then converted to printing forms. These consist of offset printing plates produced, for example, by means of halftone films.
The brightness steps or tone value steps of a printed color are obtained in the case of autotypical multicolor printing by representing the original on the printing form of each color separation by a great number of printing screen dots having a different size or area coverage per unit of printing area. Each surface coverage corresponds to a brightness step, and the sum of all brightness steps gives the tone value scale which is defined at the dark end by a screen dot area of 100%, corresponding to a unit area uniformly covered with printing ink, and at the light end by a screen dot area coverage of 0% of the whitest color of the medium (e.g., paper). On the other hand, the so-called color shades are obtained by the precise overprinting of the screened color separations of the original, on the basis of a so-called autotypical color mixing, which is a combination of additive and subtractive color mixing, since the screen dots are located partially one over the other and partially beside one another on the paper. By maintaining recommended and partially standardized angles at which the screen dots are printed one over the other in the screens of the different printing plates it is brought about that no substantial color variations can be caused by varying proportions of superimposed and side-by-side screen dots.
In modern multicolor offset printing machines, the inks are printed in rapid succession onto the paper, a separate inking unit being provided for each ink. If, for example, 10 brightness steps are provided for each printing ink, 1000 different shades of color can be obtained with three printing inks. The reproduction of a color shade depends essentially on two factors, namely, on the thickness of the printing ink film on the paper, on the one hand, and, on the other hand, on the above-mentioned area coverage of the screen dots. For the control of these factors, the inking units of the printing mechanisms of the multicolor offset printing machine are provided each with an ink fountain and a plurality of regulators in the form of so-called ink keys or ducts whereby the supply of ink to adjacent color zones (or longitudinal strips) of the printing forms or paper can be adjusted individually. As a rule, an increase of the ink feed is associated both with a vertically oriented increase of the ink film thickness and with a horizontally oriented spreading or increase of the area coverage of the screen dots, while a reduction of the ink feed leads to a corresponding reduction of the ink film thickness and the area coverage of the screen dots.
For the control of the printing processes, mainly three aids are used today. The first aid consists in performing optical density measurements by means of manually operated or automatic densitometers at preselected measurement areas in the form of screen patches and/or solid patches, i.e., surfaces completely covered with printing ink. The screen patches and solid patches can be parts of the printed picture itself, or they can be produced by providing separate patches on the printing form. The densitometric evaluation performed on a solid patch results in a value to be referred to hereinafter as the solid density, while the densitometric evaluation of a screen patch results in a value to be referred to hereinafter as the screen density. The density values give information on changes in the ink film thickness or on the size of the screen dots. The second aid consists in providing the printing forms with special control elements which consist of different sizes of screen dots and different sizes of micromeasuring elements which disappear or are retained in the printing and thus permit a direct quantitative evaluation of the variation of the screen dots or their size. Separate density measurements are not necessary, but they can be performed additionally. The control elements are provided, like the measuring patches, preferably at the top or bottom margin of the printing form or print, special control elements or measuring patches being best associated with each inker regulator and thus with each color zone of the printed picture, and furthermore special control elements or measuring patches are associated with each color separation. Lastly, the third aid consists in the use of semiautomatic or fully automatic control devices, especially in conjunction with multicolor offset printing presses. These control devices are based on the principle of using manually operated or automatically operating densitometers to measure the screen densities and/or solid densities of printed screen patches and/or solid patches, comparing the measured densities with standard values or tolerance ranges and, in the case of departures of the determined densities from the standard values or tolerances ranges, to operate the actuators of the inking mechanisms such that the measured densities will return to their proper value or come within the tolerances. In contrast to the other two aids, whose main purpose is to check the printing result, the third aid is also aimed at changing the printing result if the measured values differ from the specified values. In automatic control apparatus, this is accomplished as follows: the densities obtained with densitomers are fed to an electronic data processing apparatus equipped with microprocessors, compared in this apparatus with preselected values or tolerance ranges and, if the differences are out of tolerance, they are used for the computation of an actuating signal which serves for the automatic adjustment of the corresponding regulator, which is, for example, a key which can be turned by a stepper motor.
In making a multicolor print, the pressman can therefore proceed essentially as follows:
The pressman first begins to print at a low ink feed in order to coordinate the, for example, four printing inks such that a perfect fit results, which is important to the sharpness of the printed picture. The pressman then attempts to control the finished print result by controlling the feed of the printing inks to the color zones by means of the regulators such that it will approach the original on hand, which can be a test print, known in technical language as a proof, or it can also be the same model that served for the production of the color separations. The matching of the print result to the original is performed mainly by feel and on the basis of visual comparison of the original and the print, i.e., by subjective criteria. By constant corrections of the regulators of the inking mechanisms, the attempt is made to come ever closer to the original, or to keep the results obtained constant through the duration of the printing run. It is no more possible to achieve complete visual identity between the print and the original than it is to achieve uniform printing results over a long period of time. What color and shade differences remain is subject to a great extent to the subjective perceptions of the pressman or the client, who often is present at the beginning of a production run. Control of the printing result is therefore time-consuming and inaccurate.
To rule out the subjectivity of impressions perceived in the inspection of the printing result, the pressman can use the above-mentioned measuring patches and control elements and evaluate them continually. Alternatively, the pressman can provide a semiautomatic or fully automatic control system and intervene to help it occasionally when even the control system is no longer able to maintain identity between the original and the print.
All these measures and aids for the attainment of a uniform printing result suffer from three main disadvantages.
First, all that is available for corrective intervention in the printing process is the inking mechanisms of the multicolor printing machine or the sum of the regulators controlling the feed of ink. Therefore, the ink film thicknesses and screen dot areas can only be changed all together, not independently of one another, since varying the setting of a key or the like results not only in a change in the ink film thickness, but always also in a change in the area of the screen dots in the color zones in question. As a result, both the measured values of the solid densities and the measured values of the screen densities vary whenever a corrective intervention is made in the printing process.
Secondly, there is no clear or constant relationship between changes in the ink film thickness and changes in the dot area coverage, since the correlation between changes in screen densities and changes in solid densities constantly varies in the course of a printing process. It is to be noted that changes in the ink film thickness have a great influence on the brightness steps within a given printing color and a slight influence on the color shades formed by the interaction of several printing inks, while the reverse is true of changes in the area coverage of the screen dots. Any kind of fixed relationship or correlation between these changes has hitherto been found only for periods of time to be measured in minutes, i.e., no more than short-term relationships. For the long-term relationships measured in hours, which are especially important for production runs, however, considerable variations are found in the correlations between changes in the solid densities and the screen densities. The reason for this is to be found in the rheology of the printing inks and thus in their tendency to form screen dots of different size under the influence of heat and the feed of dampening water. However, oxidation processes and other phenomena also have an effect on the correlations. This can go so far that, in one border zone in a long production run, for example, only comparably small changes in the screen dot area coverage can be produced even by very great changes in the printing ink feed combined with a great change in the ink film thickness, while in another border zone in the same long production run, small changes in the ink feed and ink film thickness produce great changes in the area coverage of the screen dots. In these cases the most important factor to be heeded in the printing process, namely the color balance, is changed or affected differently. As a result, the action of the above-described aids, especially the control methods and apparatus (although the latter are of considerable help to the pressman since they operate on the basis of objective criteria), are actually based on one of two of the heretofore possible compromises, namely the establishment of either narrow or comparatively great tolerance ranges of screen densities and/or solid densities. If narrow tolerances are established, the color balance can be held in the short term to a sufficiently constant value. The printing run, however, must be frequently interrupted, because changes in the correlation between the screen densities and solid densities will in the long run soon cause departures from the tolerances or the control apparatus will become uncontrollable, because adjustments of the regulators will no longer permit the variation of the area coverage of the screen dots that is needed to sustain the color balance. If, however, wide tolerances are established, control of the color balance is virtually abandoned because the human eye is very sensitive to color shade changes based on changes in the screen dot area, and therefore, on the basis of the present knowledge, the screen densities and the dot areas should remain as unvarying as possible. Overall, therefore, the achievement of a uniform printing result is still today encumbered by many deficiencies.
Thirdly, considerable problems are encountered with regard to the shape, arrangement, number and size of the measuring patches. The regulators of common printing presses have widths between 30 mm and 40 mm, so that color zones of corresponding width are formed, while a great number of regulators and color zones are arrayed contiguously with one another. As a result, all of the measuring patches have to be contained within a width of 30 mm to 40 mm, inasmuch as each individual color zone must be examined, evaluated and regulated independently of adjacent color zones, as is desirable in modern printing presses.
The size and the arrangement of the measuring patches are subject in practice to two limitations. On the one hand they must have a certain minimum size to enable the measuring spot of a densitometer to be situated at least for a period of time completely within each measuring patch, even when the measurements are made on continuous-feed offset paper (roll paper) moving at high speed instead of a sheet that is not moving (sheet-fed offset paper). On the other hand, the areas of paper that carry the measuring patches are cut off at the end of the printing process, so that they constitute waste which has to be minimized for reason of economy.
In normal four-color printing using three chromatic inks (magenta, cyan and yellow) and one achromatic ink (black), if measuring patches in the form of screen patches as well as measuring patches in the form of solid patches are to be used, at least six measuring patches, but preferably eight, must be provided in each of the color zones for the chromatic inks, so that the achromatic ink can also be controlled. Furthermore, additional control means in the form of microline patches, balance patches, trapping patches or the like must be present, which are not needed for the regulation, but are useful in analyzing the printing. At least 10 measuring patches and control elements would be desirable for each color zone.
On high-speed web printing presses the measuring patches should have a width of 6 mm to 8 mm, to obtain reliable measurements. If ten measuring patches are used, this would require space amounting to 60 mm to 80 mm in width, which is more than about twice the actual width of a color zone. If the ten measuring patches were arranged in a double row, the amount of waste would nearly double, which is undesirable for economic reasons alone. Until now, therefore, regulation has been performed with screen patches alone or with solid patches alone, so that only a total of six measuring patches are needed per color zone, and all measuring patches can be contained in a single row.
It is the object of the invention to develop a new strategy for the achievement of uniform printing results, and to design the method and the control apparatus of the kinds specified above so as to permit a flexible control and regulation of the printing process, yet one subject to close tolerances as regards color balance.
It is another object of the invention to propose an aid for the constant control and supervision of the printing result in the form of a set of single color strips for the regulation of multicolor offset printing presses such that no space problems will result in the simultaneously printed print control strip, and that little waste of the printed paper will be involved, even though the control is achieved with the aid of screen patches as well as with the aid of solid patches. Moreover, it is to be possible to provide additional measuring patches or control elements without thereby impairing the evaluation of the measuring patches intended for the regulation of the printing results.
Lastly, it is an object of the invention to propose an additional aid for the control of a multi-color printing press in the form of an apparatus to permit a visual determination of the color balance in the printing results. This apparatus is furthermore to help the pressman to determine the degree of difficulty involved in the printing of a picture, and to establish reasonable tolerance ranges for the solid densities and/or screen dot sizes and/or selected equations*, on the basis of the particular economic and technical possibilities involved. 6 *Also referred to herein as "selected relationships."